Green’s Dictionary of Slang

1. a phr. denoting acquiescence, ‘that’s fine’.

1976		D. Ireland Glass Canoe (1982) 62: ‘I’ll give you the drum.’ ‘Dead set.’.
1979		Lette & Carey Puberty Blues 9: Here yar. Have me lunch. Dead-set, I’m not hungry.
1985		R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 150: ‘Some of these blokes wouldn’t get a ride on a ghost train’ ‘Dead set?’ ‘Dead set, Les’.
1992		M. Coleman Fatty 114: ‘Paul, mate, dead set you were the best player on the field today by miles’.
2018		R. O’Neill ‘Ocker’ in The Drover’s Wives (2019) 182: Dead set, it was the snake!

2. a general negative phr.

1979		Lette & Carey Puberty Blues 84: Oh deadset. I’m not gunna walk all the way.
1998		Bug (Aus.) 16 Aug. 🌐 We have grown accustomed to the faces of rugby league spreading bullshit thicker than a National Party supporter growing a crop of grass. But deadset, just how much room is there up Kneel WitLacker’s arse?